Heart rate variability (HRV) has been used as a noninvasive index of parasympa-thetic tone. Based on our previous data on parasympathetic studies, we proposed the hypothesis that HRV response to both the sympathetic & parasympathetic stim-ulation/blockade display a variance which is dependent upon the initial level of autonomic tone & the intensity of stimulus. This study will test that hypo-thesis. Measurements of HRV during stepwise changes in above conditions will be plotted to test the hypothesis.